Abstract

Article Series “Dual-Energy CT: What about Radiation Dose?” (2/3) – Dual Energy vs. Single Energy Dose Comparison Studies

posted by U. Joseph Schoepf, M.D. | May 13, 2011

This article is second part of the three-article series on “Dual Energy CT: What about Radiation Dose?” and discusses comparative studies on radiation exposure with different techniques.

Dual Energy vs. Single Energy Dose Comparison Studies

Until recently, most studies comparing radiation doses of DECT and single energy CT were performed using DSCT scanners, with comparatively sparse information available in the literature on the radiation doses involved with the other above mentioned techniques for acquiring DECT data. In general, considerable caution is in order when interpreting comparative reports on radiation exposure with different techniques. A multitude of studies indiscriminately report radiation doses with various routine techniques and conclude that one or the other method results in “higher” or “lower” radiation, while no attention is paid to normalization of image quality, signal / noise ratio, or DLP.

In a recently published phantom study by Schenzle et al. such normalization was indeed performed and the authors reported the feasibility of DECT without increasing radiation dose [20]. Moreover, they observed no significant difference in image noise, but showed that the contrast to noise ratio (CNR) could be doubled with optimized DECT reconstructions. Specifically, Schenzle et al. equipped an anthropomorphic Alderson phantom with thermoluminescent diodes (TLDs), and scanned its chest with a 1st generation DSCT system in dual energy mode at 140 and 80 kVp with 14 x 1.2 mm collimation as well as on a 2nd generation DSCT system at 140 kVp and 100 kVp with selective photon shielding at 128 x 0.6 mm collimation. For dose comparison with single energy CT, the investigators obtained reference examinations at 120 kVp with 64 x 0.6 mm collimation at an equivalent CT dose index of 5.4 mGy*cm.
In this phantom study, the authors report effective dose measurements with TLDs, that were equal for both DSCT systems in dual energy mode at 140/80 kV and 140/100 kV with selective photon shielding, when compared to the single energy 120 kV reference examination [2.61 mSv (1st generation DSCT in DECT mode), 2.69 mSv (2nd generation DSCT in DECT mode), and 2.70 mSv (2nd generation DSCT in single-energy mode)]. Moreover, the image noise was reported similar for all three of the different imaging techniques. The selective photon shielding evaluated in this study is based on filtering techniques, aimed at further spectral separation and thus higher discriminatory abilities for tissue characterization of dual-energy CT acquisitions. Similarly, in a phantom study performed on a DSCT system, Yu et al. showed that images blended from low and high tube-potential data yield similar or even better iodine CNR than that of typical 120 kV images acquired at the same radiation dose [21].

An application which currently receives especially high scrutiny vis-à-vis radiation dose issues is cardiac CT. DECT has shown distinct advantages for comprehensively evaluating coronary artery morphology and the myocardial blood supply [8-12, 22-25]. Initially, the use of dual-energy CT for cardiac applications required decreasing the temporal resolution from 83 ms to 165 ms [8]. This was not in keeping with the tenet that obtaining ancillary information on the myocardial blood supply must not detract from the full diagnostic performance of coronary CT angiography for stenosis detection.
Meanwhile, however, this limitation has been successfully addressed and the full temporal resolution of 83ms (1st generation dual-source CT) or 75ms (2nd generation dual-source CT) is now available for cardiac dual-energy CT studies [26].
Regarding radiation exposure at cardiac DECT, a recently published study by Kerl et al. compared dose levels at coronary CT angiography between dual-energy CT, 1st generation DSCT in single-energy mode, and 16-slice single source CT. This investigation reports lower dose levels for DECT and DSCT than for 16-slice CT [27]. In addition, the investigators conclude that DECT delivers significantly lower dose than does regular DSCT, but still maintains comparably good diagnostic image quality. The authors report significantly higher CNR with DSCT in dual-energy mode compared to both DSCT and 16-slice CT and attribute this observation to the low –kV data used in the DECT reconstruction algorithm. The superior CNR found with low-kV protocols is a result of the increase in the photoelectric effect at lower tube voltages, particularly in examinations of structures with a high anatomic number, such as iodinated contrast material [28]. During evaluation, DECT permits alteration of the balance between images acquired at low kV and high kV data for each study. For example, inclusion of more of the low kV data set results in increased attenuation of contrast material in the coronary arteries. Consequently, this ability to enhance contrast attenuation could facilitate a reduction in the total amount of contrast medium required for this and other contrast medium enhanced CT applications without incurring higher image noise levels involved with single-energy low kV imaging.

As mentioned above, very limited information is available in the literature regarding radiation doses associated with multi-energy imaging on platforms that are not based on dual-source CT technology. Ho et al. observed two to three times higher doses for DECT [25] using a single-source system and rapid voltage switching (22.5 to 36.4 mSv for DECT vs. 9.4 to 13.8 mSv for single-source CT). This group did not perform normalization of image noise or dose; thus the lower energy spectrum was obtained with the same tube current time product as the single energy scan.
In another study that did perform normalization to equivalent low-contrast detectability, Li et al. observed an additional dose of 14% in the body and 22% in the head using DECT with rapid kV switching in comparison to that of single energy CT [29]. At this point, no study has systemically evaluated the radiation dose of multiple energy image acquisitions using a single source dual-layer detector CT system, or combining image data from two consecutive 270° rotations. Furthermore, clinical studies specifically comparing the dose efficiency of different DECT systems are lacking.

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